Heat transfer fluids (e.g., coolants) for internal combustion engines (“ICEs”) are known. Such fluids commonly contain about 50% water and 50% ethylene glycol (by weight) with trace amounts of additives, including corrosion inhibitors. However, the ICE may be obsolete within the coming decades. Fuel cells have emerged as a potential replacement. In general, a fuel cell is an electrochemical device that converts the chemical energy of a fuel into electrical energy. They provide several advantages over ICE. Fuel cells are more efficient in extracting energy from fuel (e.g., 60-70% efficiency as compared to 40% for turbodiesel engines and 30% for gasoline engines). Further, fuel cells are quiet and produce negligible emissions of pollutants. Also, the primary fuel source for the fuel cell is hydrogen, which is more readily available than ICE fuel sources (e.g., gasoline). However, replacement of the ICE with fuel cells may require the concomitant replacement of known heat transfer fluids.
Typically, a fuel cell consists of an anode (a positively charged electrode), a cathode (a negatively charged electrode) and an electrolyte in between the two electrodes. Each electrode is coated with a catalyst layer. At the anode, a fuel, such as hydrogen, is converted catalytically to form cations, which migrate through the electrolyte to the cathode. At the cathode, an oxidant, such as oxygen, reacts at the catalyst layer to form anions. The reaction between anions and cations generates a reaction product, electricity and heat.
The current produced in a fuel cell is proportional to the size (area) of the electrodes. A single fuel cell typically produces a relatively small voltage (approximately 1 volt). To produce a higher voltage, several fuel cells are connected, either in series or in parallel, through bipolar plates separating adjacent fuel cells (i.e., “stacked”). As used herein, a fuel cell assembly refers to an individual fuel cell.
The most common fuel and oxidant used in fuel cells are hydrogen and oxygen. In such fuel cells, the reactions taking place at the anode and cathode are represented by the equations:Anode reaction: H2→2H++2e−  (1)Cathode reaction: ½ O2+2H++2e−→H2O  (2)The oxygen used in fuel cells comes from air. The hydrogen used can be in the form of hydrogen gas or a “reformed” hydrogen. Reformed hydrogen is produced by a reformer, an optional component in a fuel cell assembly, whereby hydrocarbon fuels (e.g., methanol, natural gas, gasoline or the like) are converted into hydrogen. The reformation reaction produces heat, as well as hydrogen.
Currently, there are five types of fuel cells, categorized by their electrolyte (solid or liquid), operating temperature, and fuel preferences. The categories of fuel cells include: proton exchange membrane fuel cell (“PEMFC”), phosphoric acid fuel cell (“PAFC”), molten carbonate fuel cell (“MCFC”), solid oxide fuel cell (“SOFC”) and alkaline fuel cell (“AFC”).
The PEMFC, also known as polymer electrolyte membrane fuel cell, uses an ion exchange membrane as an electrolyte. The membrane permits only protons to pass between the anode and the cathode. In a PEMFC, hydrogen fuel is introduced to the anode where it is catalytically oxidized to release electrons and form protons. The electrons travel in the form of an electric current through an external circuit to the cathode. At the same time, the protons diffuse through the membrane to the cathode, where they react with oxygen to produce water, thus completing the overall process. PEMFC's operate at relatively low temperatures (about 200° F.). A disadvantage to this type of fuel cell is its sensitivity to fuel impurities.
The PAFC uses phosphoric acid as an electrolyte. The operating temperature range of a PAFC is about 300-400° F. Unlike PEMFC's, PAFC's are not sensitive to fuel impurities. This broadens the choice of fuels that they can use. However, PAFC's have several disadvantages. One disadvantage is that PAFC's use an expensive catalyst (platinum). Another is that they generate low current and power in comparison to other types of fuel cells. Also, PAFC's generally have a large size and weight.
The MCFC uses an alkali metal carbonate (e.g., Li+, Na+ or K+) as the electrolyte. In order for the alkali metal carbonate to function as an electrolyte, it must be in liquid form. As a result, MCFC's operate at temperatures of about 1200° F. Such a high operating temperature is required to achieve sufficient conductivity of the electrolyte. It allows for greater flexibility in the choice of fuels (i.e., reformed hydrogen), but, at the same time, enhances corrosion and the breakdown of cell components.
The SOFC uses a solid, nonporous metal oxide as the electrolyte, rather than an electrolyte in liquid form. SOFC's, like MCFC's, operate at high temperatures, ranging from about 700 to about 1000° C. (1290 to 1830° F.). The high operating temperature of SOFC's has the same advantages and disadvantages as those of MCFC's. An additional advantage of the SOFC lies in the solid state character of its electrolyte, which does not restrict the configuration of the fuel cell assembly (i.e., an SOFC can be designed in planar or tubular configurations).
The final type of fuel cell, known as AFC, uses an aqueous solution of alkaline potassium hydroxide as the electrolyte. Their operating temperature is from about 150 to about 200° C. (about 300-400° F.). An advantage to AFC's is that the cathode reaction is faster in alkaline electrolytes than in acidic electrolytes. However, the AFC is very susceptible to contamination, so it requires pure reactants, i.e., pure hydrogen and oxygen.
In general, the reactions that take place within the fuel cell assembly (i.e., the electrochemical reaction and the reformation reaction) are exothermic. However, the catalyst employed in these reactions is sensitive to heat. To perform optimally, fuel cells should be maintained at a certain temperature that is nearly uniform across each cell in the stack. For example, at high temperatures, the catalyst may be destroyed, while at low temperatures, ice may form within the fuel cell assembly. Thus, to accommodate such temperature requirements, heat transfer compositions are needed.
Known heat transfer compositions are not amenable for use in fuel cell assemblies. Conventional heat transfer fluids contain corrosion inhibitors, which are generally metal or organic acid salts. Such salts exist as ions in solution. The presence of significant amounts of positive and negative ions in solution provides a path for a “stray electrical current.” Such stray current must be limited for several reasons. First, it may cause electrical shock hazards to the fuel cell operator. Second, such stray current may generate highly explosive hydrogen gas in the cooling system from hydrolysis. Lastly, a significant portion of the electricity generated by the fuel cell may be shorted through the fluid, rather than going to power production, thereby decreasing the efficiency of the fuel cell assembly. Thus, heat transfer fluids used in a fuel cell application must have lower electrical conductivities (i.e., higher electrical resistance) than those used in an ICE application.
In addition to electrical resistivity, there are additional considerations involved in the development of fuel cell heat transfer fluids. One consideration relates to their application. Fuel cell heat transfer fluids in an automotive application will likely be exposed to metals different from those in an ICE. For example, fuel cell assemblies are expected to contain stainless steel, some aluminum alloys, specially coated aluminum and insulating polymers, whereas ICE contain cast iron, steel, brass, solder and copper. Thus, the fuel cell heat transfer fluids must accommodate different types of metals. Another consideration relates to the physical properties of the heat transfer fluid. In fuel cells, the heat transfer fluid must be able to flow through the assembly in order to maintain uniform temperature across the catalyst layer. This depends on the heat transfer fluid's freezing point, vapor pressure, viscosity, pumpability and laminar flow. In addition to these properties, the ability of the heat transfer fluid to adsorb gases affects the conductivity of the heat transfer fluid. As a final consideration, fuel cell heat transfer fluids, like known heat transfer fluids, must be cost effective.
In general, water or deionized water have been used as the heat transfer fluid in fuel cell applications. See, U.S. Pat. Nos. 5,252,410; 4,344,850; 6,120,925; and 5,804,326. However, there are several disadvantages to using water as a fuel cell heat transfer fluid. First, a fuel cell may be exposed to extremes in environmental conditions, e.g., broad ranges in temperatures. For example, when the operating temperature of the fuel cell falls below the freezing point of water, the volumetric expansion of water may cause severe damage to the fuel cell. In addition, water may be corrosive to the different metals that are used in fuel cell applications. As a result, inorganic and/or organic inhibitors would be needed to provide long term corrosion protection. However, such inhibitors may change the electrical resistance of the heat transfer fluid. Lastly, the electrical conductivity of water may change over time as a result of accumulating degradation contaminates, by-products, and rust. Each of the above is detrimental to fuel cell performance.
Efforts at maintaining the temperature above the freezing point of the water heat transfer fluid have focused on the design of the fuel cell assembly. For example, U.S. Pat. No. 6,248,462 B1 (“the '462 patent”) discloses a fuel cell stack that contains a cooler plate interspersed throughout the fuel cell stack. The cooler plate circulates an antifreeze solution through its channels. Such cooler plate prohibits the diffusion of antifreeze into the rest of the fuel cell stack. While the cooler plate addresses the first problem associated with using water as a heat transfer fluid, it fails to obviate them all. Moreover, the addition of such a cooler plate to the fuel cell stack increases the overall weight and volume of the fuel cell stack.
Efforts to address these shortcomings have focused on the development of fuel cell assemblies that accommodate known antifreezes. For example, U.S. Pat. No. 6,316,135 B1 and International Publication No. WO 01/47052 A1 disclose a fuel cell assembly using only antifreeze as the heat transfer fluid. Such fuel cell assemblies contain certain “wetproofed,” i.e., substantially hydrophobic, components. The design of these assemblies prevents the antifreeze from contaminating the electrolyte and the catalyst, thereby eliminating the need to isolate the antifreeze from the components of the fuel cell assembly (e.g., in a cooler plate). As a result, fuel cell stacks can be made having lower weight and volume than those disclosed in the '462 patent. However, such fuel cell assemblies have several problems, including antifreeze contamination and reduced cooling effectiveness caused by the wetproofed materials.
New heat transfer fluids have also been developed. For example, each of U.S. Pat. Nos. 5,868,105; 6,101,988; 6,053,132; and 6,230,669 disclose a heat transfer fluid that is a substantially anhydrous, boilable liquid having a saturation temperature higher than that of water. The disclosed heat transfer fluids have a minimum content of water, for example, less than 5% by weight. An example of one such heat transfer fluid is propylene glycol with additives to inhibit corrosion. The use of propylene glycol as a heat transfer fluid suffers limitations. One important limitation lies in its viscosity. At low temperatures, propylene glycol is highly viscous. This reduces its flow through the fuel cell assembly, and consequently, its heat removal efficiency. The end result is a decrease in the efficiency of the fuel cell assembly.
Mixtures of water and alcohols have also been used as fuel cell heat transfer fluids. See, e.g., Japanese Patent Laying-Open Gazette No. 7-185303. Such mixtures suffer from deficiencies resulting from solvent vaporization. Alcohols, like methanol, may cause some of the heat transfer fluid to vaporize into the cooling layer. Such vaporization raises the pressure of the cooling layer, thereby preventing the heat transfer fluid from flowing at a constant rate through the fuel cell assembly. This effects the ability of the heat transfer fluid to maintain a uniform temperature across the catalyst layer.
Other fuel cell heat transfer fluids have also been used. For example, water-glycol mixtures, Thenninol D-12 (which is a hydrotreated heavy naphtha (petroleum)) and dielectric fluids (e.g., mineral oils and silicone oils) have been used in fuel cells. See, e.g., U.S. Pat. Nos. 5,565,279; 5,252,410; 5,804,326; and 6,218,038. The fuel cell heat transfer fluid disclosed in International PCT Publication WO 01/23495 comprises water, glycol and corrosion inhibitors. Each of the heat transfer fluids above has deficiencies, including flammability and increased conduction (i.e., reduced resistivity).
Thus, there remains a need for a heat transfer fluid composition that resists corrosion, freezing, vaporization and gas adsorption, while at the same time, provides long lasting performance and high electrical resistance.